Wood materials and shearwalls of older light-frame residential structures Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2r36v074t

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  • Light-frame construction practices and materials have changed greatly over the past 100 years. Contemporary research has focused on modern construction; thus, we know a great deal about the behavior of modern lightframe buildings under lateral forces. However, there are many light-frame buildings that were built prior to the introduction of modern building codes and material standards, and these buildings are still in service. The material and performance databases for these older structures are limited, so risk assessment and condition assessment are challenged for seismic or wind events. The project objective is to establish a basis for probabilistic assessment of the seismic performance of older construction by examining the performance of shearwalls, connections, and wood materials from older light-frame buildings Nineteen structures built between 1900 and 1970, scheduled for demolition, were sampled for material and connection tests as well as full-size shearwall tests. The scope of tests for each source structure was based on the availability of full-size shearwalls and the type of sheathing material used in the structure. Two exterior sheathing types were found in the source structures, horizontal plank sheathing and plywood. Wood lath-and-plaster was the characteristic interior wall covering in buildings of this era. Specific gravity was determined, and embedment tests were performed on the wood framing and sheathing materials. Bending-yield tests were performed on the sheathing nails (typically 0.113 by 2.5- in.), and lateral single-nail connection tests were performed on extracted connections. Full-scale shearwall racking tests were done both monotonically and cyclically using the basic CUREE loading protocol. The average specific gravity of the wood materials was 0.46. The material extracted from the source structures had an embedment strength that was statistically similar to the National Design Specifications (NDS) table value for a specific gravity of 0.46 (4.0 ksi). The results of the nail bending-yield test showed no significant change over time. Nails had average bending-yield strength of 97.3 ksi, which is similar to the NDS stated value of 100 ksi. In general, the connection tests showed agreement with the European Yield Model (EYM) equations for connection strength. The full-size shearwall capacities were in agreement with known values for walls with each type of sheathing. Based on the limited testing done in this study, no adverse effects due to age and service life were observed. The materials and assemblies performed according to modern standards for new construction. Insect damage and fungi deterioration were present in many of the structures, and because these conditions were avoided as much as possible, no inferences are made regarding the effects of insect and fungi damage on lateral shear strength. These tests show that a structure built in the early 1900’s will meet modern design expectations as long as the material has been kept dry and free of damage due to insects. The principal threats to hazard performance observed during this study were the construction practices in the early twentieth century. Most of the source structures had no anchorage to the foundation, shearwalls were connected to roof diaphragms with limited toe nail connections, most structures were sheathed with horizontal planks, and many of the source structures had few walls that met the modern prescribed aspect ratio for structural shearwalls of 2:1 for full table design capacity. The results of this research can be integrated with the Federal Emergency Management Administration (FEMA) document on seismic rehabilitation for buildings.
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